1897 Table VI. Apparent Helix Contents of @-LactoglobulinA in Water-2-Chloroethanol Mixtures ~
2-Chloroethanol, VOl
z
~~
~
Apparent per cent right handed bo Hiss
0 10 20 30
11 10 36 46
40
50 53 58 64
60 80 100
CY
26 26 42 48
53 56
60 65
helix from H225
11 11 37 45 47
50 52 57
equation parameter^.^^^^^ The results are summarized in Table VI. We see that, by all criteria, the apparent contents of a helix increase sharply between 10 and 20% 2-chloroethanol. Above this solvent composition, a slow rise in a a helix contents continues, reaching a maximum of about 65% in pure 2-chloroethanol, in a manner similar to the earlier observations in acidic methanoL6 Comparison of the HIg3and H225 values obtained in the presence of 2-chloroethanol (Table VI) shows that, according to the criteria of Shechter and B l o ~ t , ~the ’ optical rotatory dispersion properties of P-lactoglobulin in 2-chloroethanol cannot be accounted for in terms of a-helical and unordered regions alone, but that another non-a-helical ordered structure persists. When the S-S bridges in P-lactoglobulin are broken by S-sulfonation, the H I g 3and H225values obtained in methanol become 87 and 88, respectively, indicating the disappearance of the “third” structure; this suggests that there is in this protein a structurally stable region constrained by an S-S bridge. In the earlier study with methanol,6 the conclusion had been reached that this stable region is rich in ,t?structure. It is interesting to note that the large electrophoretically (57) E. Shechter and E. R. Blout, Proc. Narl. Acad. Sci. U. S., 51, 695. 794 (1964). .. (58) J.’P.Carver, E. Shechter, and E. R. Blout, J. Am. Chem. Soc., 88, 2562 (1966).
20 Volume
40 60 BO Per Cent of Chloroelhnnol
100
Figure 5. Dependence of aa,bo, [ m f ] r 3 ~[mf]199, .6, and [el,, of Plactoglobulin A on 2-chloroethanol concentration in the mixed solvents of water with 2-chloroethanol.
immobile tryptic peptide of P-lact~globulin~~ gives an infrared spectrum in the amide I band region typical for an antiparallel-chain pleated-sheet structure.56 The dependence of the preferential binding of 2chloroethanol to 6-Lg A on the concentration of the former is very similar to that observed in the case of the w a t e r 4 loroethanol-bovine serum albumin system.6o It is, however, quite different from that found with other organic solvents, for example, with P-Lg A in ethylene glycol and methyl Cellosolve,61 and bovine serum albumin in glycerol.60 In addition, this concentration dependence of preferential interaction with solvent components does not seem to have any simple relation to the conformational change in 6-Lg A induced by the same mixed solvents. Further investigations on the effect of this and other solvent components on several proteins are now in progress. Acknowledgment. This work was supported in part by Grant GB-5186 from the National Science Foundation and Grant G M 14603-02 from the National Institutes of Health. (59) R.Townend, Arch. Eiochem. Eiophys., 109,l (1965). (60) J. Stauff and K. N. Mehrotra, Kolloid-Z., 176, 1 (1961). (61) H.Inoue and S . N. Timasheff, in preparation.
Communications to the Editor Reactions of Nonrigid Systems Sensitized by Anthracene and Substituted Anthracenes’ Sir: Evidence has been presented that anthracenes sensitize reactions of rigid systems by energy transfer from their second triplet states.’ We wish now to report examples of Tz energy transfer to nonrigid systems where the proposed “nonvertical” excitations are more likely to occur. (1) The Role of Second Triplet States in Solution Photochemistry, 11. For the previous paper in this series, see R. S. H. Liu and J. R. Edman, J. A m . Chem. Soc., 90,213 (1968). (2) See, e.g., G.S . Harnmond, Kagaku To Kogyo (Tokyo), 18, 1464
(1965).
The product composition of photosensitized dimerization of butadiene is known to vary with the triplet excitation energy of the sensitizer. With anthracene and 9,lO-dibromoanthracene (DBA), the dimer compositions are anomalous in that they agree with those of high-energy sensitizers (>60 kcal/mole). The dimer compositions produced by many other anthracenes (Table I) are likewise anomalous, suggesting sensitization occurs by energy transfer from the Tz states of the anthracenes. The photoisomerization of 1,3-pentadiene (piperylene), for which the concept of nonvertical excitation (3) R. S . H.Liu, N. J. Turro, and G. S . Hammond, J. A m . Chem. SOC., 87, 3406 (1965), and previous papers in the series.
Communications to the Editor
1898
A common intermediate in all three reactions is clearly
Table I. Butadiene Dimer Composition Sensitized by Anthracenesa Sensitizer
ET,kcal/moleb TI Per cent Ch T 2
74.4 Anthracene 73.5 2-Methylanthracene 9-Methylanthracene ... 9-Methyl- 10-chloroanthracene 9-Methyl- 10-chloromethyl... anthracene 1,5-Dichloroanthracene 72.6 9,lO-Dichloroanthracene ... 9,lO-Dibromoanthracene ... 9,9’-Bianthryl ... .
I
42.5 40.6 40.6
95, 84O 95 95 92 95
...
.
...
40.7 40.2 40.2
94 95 94, 97c 96
...
indicated. The photostationary state with DBA as sensitizer (% trans = 57)7*8falls well within the range for sensitizers of energy above 60 k ~ a l / m o l e . ~ The photosensitized dimerizationg of acrylonitrile gives a mixture of cis- and trans-1,2-dicyanocyclobutanes. The ratio of the product isomers (Table 111) Table 111. Photosensitized Dimerization of Acrylonitrile (AN).
Neat butadiene in Pyrex tubes irradiated around a 200-W Hanovia medium-pressure Hg lamp with Pyrex housing. See literature quoted in ref 1. c Reference 3.
Sensitizer
a
Xanthone Acetophenone Triphenylene Benzophenone
was also invoked, was reinvestigated with emphasis on substituted anthracenes (Table 11). The anthracenes fail to follow a monotonic decrease of quantum yield with sensitizer energy, suggesting that T2 of anthracenes is, at least in part, responsible for the observed reaction. Lower values of quantum yields were obtained with double-sensitizer irradiation’ in which selective popula-
2-Acetonaphthone 1-Acetonaphthone 9-Fluorenone DBA
Table 11. Quantum Yields of Photosensitized Isomerization of Piperylenes“ (9-Fluorenone DBA)‘ 9-Bromoanthracene
+
Sensitizer Benzophenone Michler’s ketone 9-Fluorenone Benzanthrone Acridine 9,lO-Dichloroanthracene 9,lO-Dibromoanthracene 9-Methyl- 10-bromoanthracene Benzanthrone (7 X M) DBA (1 x 10-3) Benzanthrone (2.33 X (70d) DBA (1 x 10-3) Acridine (7 X DBA (1 x 10-3) Acridine (2.33 X (60)d DBA (1 x 10-3)
+ + + +
0 . 5 5 i 0 . 0 1 ~0.44i. 0.01c 0.01c 0 . 5 5 i 0 . 0 1 ~0.43
0.74 8.4 x 5.4 x 1.8 X 4.2 x 3.3 x 5.1 x
*
10-3 10-3 3.2
x
10-2
10-2 10-3
[AN], M (solvent).
Dicyanocyclobutane (tic)*
10 (B) Neat Neat Neat 10 (B) l.O(B) 0 . 3 (B) 10 (A) l.O(A) Neat Neat Neat Neat 10 (B) l.O(B) 0 . 3 (B) 10 (A)’ l.O(A) Neat
0.8 0.82d 0.82d 0.84d 0.8 1.3 2.33d 0.8 0.8 None None Noned 0.82d 0.7 1.2 2.33d 0.7 0.7
Neat
0.9
...
z @c ...
...
0.062
...
... ...
...
